https://github.com/tekezo/Karabiner-Elements/issues/1051 solution

CmdShiftLanguages.json

{
  "title": "Language toggler",
  "rules": [
    {
      "description": "Left_Shift+Left_Command language toggle (en->ru->en...)",
      "manipulators": [
        {
          "conditions": [
            {
              "type": "input_source_if",
              "input_sources": [
                {
                  "language": "en"
                }
              ]
            }
          ],
          "type": "basic",
          "from": {
            "key_code": "left_shift",
            "modifiers": {
              "mandatory": [
                "left_command"
              ]
            }
          },
          "to_if_alone": [
            {
              "select_input_source": {
                "language": "ru"
              }
            }
          ]
        },
        {
          "conditions": [
            {
              "type": "input_source_if",
              "input_sources": [
                {
                  "language": "ru"
                }
              ]
            }
          ],
          "type": "basic",
          "from": {
            "key_code": "left_shift",
            "modifiers": {
              "mandatory": [
                "left_command"
              ]
            }
          },
          "to_if_alone": [
            {
              "select_input_source": {
                "language": "en"
              }
            }
          ]
        }
      ]
    },

    {
      "description": "Right_Shift+Right_Command language toggle (en->ru->en...)",
      "manipulators": [
        {
          "conditions": [
            {
              "type": "input_source_if",
              "input_sources": [
                {
                  "language": "en"
                }
              ]
            }
          ],
          "type": "basic",
          "from": {
            "key_code": "right_shift",
            "modifiers": {
              "mandatory": [
                "right_command"
              ]
            }
          },
          "to_if_alone": [
            {
              "select_input_source": {
                "language": "ru"
              }
            }
          ]
        },
        {
          "conditions": [
            {
              "type": "input_source_if",
              "input_sources": [
                {
                  "language": "ru"
                }
              ]
            }
          ],
          "type": "basic",
          "from": {
            "key_code": "right_shift",
            "modifiers": {
              "mandatory": [
                "right_command"
              ]
            }
          },
          "to_if_alone": [
            {
              "select_input_source": {
                "language": "en"
              }
            }
          ]
        }
      ]
    }
  ]
}